When the runtime needs to turn some kinds of signals into managed
exceptions (for example: SIGINT turns into
`new ExecutionEngineException ("Interrupted (SIGINT)")`, and some
SIGFPE turn into `DivideByZeroException`, and some SIGSEGV turn into a
`NullReferenceException`) instead of unwinding the stack from inside a
signal handler it instead adjusts the normal stack so that when the
signal handler returns, execution will resume in
`handle_signal_exception`.

That means that if the runtime was in GC Safe mode when the signal
was raised, even if the signal handler code transitions to GC Unsafe
mode, by the time the `handle_signal_exception` runs, we will have
undone the GC Unsafe transition and will be back in GC Safe.

That means if the code in `handle_signal_exception` (notably
`mono_handle_exception`) calls anything that tries to do a transition
to GC Safe, we may get an assertion.

Fixes dotnet#88405